SnowBro explains how Metroid's map data works, and disproves the "Secret Worlds" myth.
METROID MAP DATA FORMAT v1.1 The death of the "Secret Worlds" myth by Kent Hansen (SnowBro) <email@example.com> Most fans of this timeless NES classic have probably heard of the so-called "Secret Worlds" of Metroid. For a long time, I maintained a site concerning these strange areas. It was my hope that they weren't just a glitch, but actually put there on purpose, as an added bonus to those who had played the game to death (and there are a lot of us). So I spent many hours creating maps and writing detailed descriptions on how to access every one of these parts of the game. Recently, however, I've come to the (sad) conclusion that the areas are simply bugs in the game engine. To understand what really happens when you access one of these areas, one has to know about the format of the level data. Each of the main parts of Metroid: Brinstar, Norfair, Tourian, Kraid's Hideout and Ridley's Hideout, have an AREA MAP. (Actually, they are all part of ONE large map.) Each location in this map contains a value which holds the room number to display at that location in the area. OK, let's take a concrete example. Here's what the map data for the first horizontal corridor in the game (where you start out) looks like: 08 17 09 14 13 | | | | | +------ This is where you get beamed down. +--------- This is where the Maru Mari (crystal ball) is. TIP: I suggest downloading my Metroid level editor (see link at the end of this file), MetEdit, to play around with this. It displays the room number of each map location, so you can see for yourself what I mean, if the info above is not clear. Also, it has a map viewer, which lets you view the entire game map. The actual ROOM DATA (the objects and enemies in the rooms) for these map locations is defined elsewhere in the ROM. (I won't get into the format of that data, since it's quite complex and you don't really need to know it, unless you're planning to make your own level editor for Metroid.) What does this mean, then? Well, it means that the game can use the same rooms more than once; it simply has to reference the appropriate room number in the map data. For example, the following sequence of values make up a long horizontal corridor: 14 14 14 14 14 TIP: MetEdit has the ability to change which room number is contained in each map location, so I suggest you change the room numbers in the start area of the game, save the changes and run the ROM in an emulator to see the full effect. Next follows an explanation of how the game code uses the map data when you're playing the game. At all times, when you're playing Metroid, there is a value in NES memory which holds the offset (pointer) into the area map. When you go one room left, the pointer is decreased by one. When you go one room right, the pointer is increased by one. When you go one room up, the pointer is decreased by 32 (32 is the width of the map). When you go one room down, the pointer is increased by 32. This way the game code can effectively fetch the room number from the current area map position and set up the room you've entered. For a graphical representation of what I just explained, check out the map in the upper left corner of Zelda; it's very similar to how the Metroid area maps look like in memory. OK, now for the "hidden area" part. You probably know about the so-called "wall-door" trick; the method that must be employed in order to access these areas. (Actually, you don't have to anymore, because there are Game Genie codes which enable you fall right through the floor and jump through walls - see end of this document.) The game programmers never intended this to be possible. So what happens when you go up or down, when you're not really supposed to be able to? Well, the map pointer will be decreased/increased as usual. But the data it points to will NO LONGER BE VALID, because the data it accesses isn't part of the area's map! This won't crash the game though; it will continue to treat the data as valid. So there you have your "secret world": a mix of invalid map locations containing who-knows-what. But wait: the map data accessed in such cases, is actually technically valid; it IS part of the game map. The problem is that the data belongs to a different area. You've probably heard of the "huge secret world" above Ridley's Hideout. Well, what really happens when you enter that area, is that you're accessing the map data from Norfair, since that is the data contained in memory directly above the map for Ridley's Hideout. I know that this may sound strange, but to make things clearer, let me explain what happens when you take an elevator from one part of the game to the other. The elevators are actually just excuses for the game code to bank-switch. As you probably know, the NES has a pretty limited amount of memory; only 32K of program ROM space. So when you take an elevator, the game uses this opportunity to switch to another ROM bank, containing the music, graphics data (since Metroid has no VROM), pointer tables, room data and other things specific to that area of the game. This explains why the "secret world" above Ridley's Hideout uses map data from Norfair, yet the rooms look just like they are part of Ridley's Hideout: the game code hasn't bank-switched. It will use the map data from Norfair, but the ROOM DATA and graphics from Ridley's Hideout! TIP: MetEdit doesn't require you to go via the elevators to change the graphics and room data; every time you enter a new room, the editor checks which area that position in the map belongs to, and sets the pointers to the data accordingly. This makes it easy to test what I just explained above: From the elevator in Ridley's Hideout, go down one room, then go to the far right, until you reach the top of a long vertical shaft. Above this point, one of the "secret worlds" is located. But by going up a couple of times you see that, WOW, this isn't a hidden zone; it's Norfair! Press spacebar to enter the map viewer, then you can see where you're actually at. Does it look like a secret world to you? And that's it. This document has probably crushed the hopeful dreams of quite a few avid Metroid gamers, but on the other hand; at least you know the truth now. Send questions, complaints or suggestions to firstname.lastname@example.org. Download MetEdit from: http://www.classicgaming.com/mdb Some interesting Game Genie codes for Metroid: SUPEAI - descend through floor a little each time you shoot up. TTXTTT - walk left through walls. EEXTTY - jump through ceilings. KAPGEI - jump in mid-air. SSAEAI - travel freely through walls when you're a ball. SSXAEI - get stuck in walls (it CAN be useful). KUSTIN - standby if you get stuck in a wall. Thanks to Dave Stewart & Jeffrey Toyes for these codes. (c) Kent Hansen 1998